In the context of artisanal measurement and cutting to size of horizontal workpieces, such as e.g. table tops and kitchen worktops composed of wood, stone or glass, besides conventional analog measuring means, electro-optical measuring apparatuses, in particular distance measuring apparatuses, are also being used more and more often.
Generic electro-optical measuring apparatuses, in particular handheld laser distance measuring apparatuses as described for example in WO 2005/083465, EP 0 738 899 and EP 0 701 702, for optically measuring distances are used nowadays in large numbers for a wide variety of applications, particularly in skilled crafts and trades and in the building and construction industry. They can be used to optically measure distances, for example between a measuring stop of the measuring apparatus and a natural target object (i.e. target object not cooperating with the measuring apparatus), within a distance measurement range of from a few centimeters to 30 meters, for example, with a very high accuracy.
In addition, the electro-optical measuring apparatus can be equipped with an angle determining unit for determining solid angles in relation to a reference coordinate system by means of which three-dimensional coordinates of spatial points can be determined and indicated. For this purpose, such a handheld distance measuring apparatus can comprise a referencing support which can be used to determine angles and changes in alignment of the distance measuring apparatus relative to an external reference object that is stationary with respect to the reference coordinate system. The spatial alignment of the distance measuring apparatus relative to the reference object is detectable by means of goniometers, in particular. Moreover, provision can additionally be made of inclination sensors for determining the alignment in relation to Earth's gravitational field vector.
In one conventional embodiment of such a measuring apparatus, optical beams modulated by means of an optical unit are emitted as transmission beams or measurement light beams toward the target object to be measured. At least some of the measurement light beams are reflected back from the surface region of the natural target object in the direction of the measuring apparatus—in contrast to a target object cooperating with the measuring apparatus, such as a cat's eye or a reflector, for example, the function of which is to completely reflect the measurement beam. By means of the optical unit, beams reflected from said surface region (in particular in a manner spaced apart from the transmission beams) are collected again and converted into an electrical signal by a receiver of the device. Known electro-optical measuring apparatuses and distance measuring methods of this type are based on a measurement of the time of flight of a temporal light pulse or a measurement of the phase shift of a laser beam reflected from the target object. In the time-of-flight measurement, the pulse time of flight from the laser light source to the target object and back to a receiver is measured, as a result of which an extremely short required measurement time can be realized.
Electro-optical measuring apparatuses and distance measuring methods are furthermore based e.g. on measurement of the phase of the reflected laser light and are based on the fact that the phase shift of the reflected laser beam or the modulation thereof relative to the emitted beam is distance-dependent. Since the laser light exhibits a beam diameter, it is always projected and correspondingly reflected in a planar fashion. An average value over the beam cross section is therefore used for the calculation of the distance.
By way of example, if parts of the laser beam impinge on an edge or corner, but other parts impinge on the areas further back, the averaged distance then lies “in the table” and is thus in other words too long. Correspondingly, in the case of the measurement of an angle (“inner edge”), the measured distance is too short since the beams reflected from the areas further toward the front concomitantly influence the average value calculation.
By way of example, if the laser beam impinges on a surface at a very shallow angle, it appears even more planar, e.g. as an elongated ellipse. During correct targeting with the center of the ellipse, although the subsequently measured distance is correct, said center can be discerned only with difficulty by the naked eye and a measurement beam can be manually aligned therewith very poorly. In addition to the measurement errors described above, therefore, another factor is that correct sighting can be made more difficult. It is exactly when such precise technology is used that it is important, however, for the target object to be easily sightable, in order then to be able to be measured exactly.
However, beams directed directly onto edges and corners to be measured that face the measuring apparatus or observer, and onto surfaces at a shallow angle can slightly miss the exact target very easily because they are alignable therewith with very great difficulty. Those additional targeting errors, in the subsequent measurement, can yield deviating distance data which are unacceptable for some applications.
If an edge or corner facing the measuring apparatus or observer is sighted, then—particularly from a relatively far distance—it can be a challenge to align the measurement beam precisely with the point to be measured at the edge or corner (in general: the edge line or the corner point). It is not possible to assess in a simple manner here whether the center of the beam is then exactly on the edge or the corner.
The measurement of an edge not at right angles, in which for example the edge line is not the outermost point of the table edge, is potentially made more difficult in addition. This is because if said outermost point is of interest for the measurement, there is the risk here of said point likewise not being discernible and sightable in a simple manner, as is the case for example for a rounded table edge, the outermost point of which is not readily ascertainable visually. The same applies to a corner which faces the measuring apparatus or observer and the corner vertex of which is intended to be sighted.
Surfaces, edges or corners to be measured that face away from the measuring apparatus or observer are likewise difficult to sight or in some instances not sightable at all, which can occur for example when measuring an averted table top edge which one would like to measure from the opposite table top edge using an electro-optical measuring apparatus.
Apart from the problems described above, moreover, a measurement result would be correct only if the visible and targeted edge also actually represented the table top end which is intended to be measured. This would be the case for a substantially non-rounded edge falling at an angle of at least 90°. However, some edge profiles are not directly measurable because they have, for instance, an edge that is chamfered, oblique (falling angle>90°) or provided with a radius of curvature, such that the outermost point of the table top end is not directly visible and thus not measurable from the measuring apparatus.
The same applies to a corner which faces away from the measuring apparatus or observer and the corner vertex of which is intended to be sighted.